High voltage transistor operation is often desirable in certain circuit applications. For example, in high fidelity audio amplifiers the power output stage requires substantial power supply voltage in order to obtain the required power output. Since the driver transistors are ordinarily directly coupled to the output transistors, they too must withstand the substantial voltages involved. The power output transistors are specially made to provide the currents and voltages required to deliver a rated power level and are therefore relatively costly. Since they are required to dissipate substantial power, they are usually made in discrete form with a shape that allows suitable heat sinking. The driver transistors are desirably of more conventional construction and most desirably are incorporated into an integrated circuit (IC) along with gain stages, protective circuitry, and other related circuit functions.
Using conventional discrete transistors and IC's at the required high voltages has led to the use of stacked transistor circuits. Two or more transistors are series connected and biased so as to divide the applied voltage between the transistors. Typically the series transistors are biased from a voltage divider that consists of series connected resistors. This sort of biasing works well for operating quiescent transistors and the current flow in the divider can be made greater than the transistor base currents to obtain stable bias. However, as the transistors are driven into conduction, less current flows in the divider and the stacking transistor base current is limited. This means that very limited current drive is available in stacked transistors. For moderate output loads this means that their voltage swing capability is also limited.